1. Field of the Invention
The present invention relates to a spectral characteristic measuring apparatus such as a spectrocolorimeter for measuring a spectral characteristic of a sample to be measured (hereinafter, called as a sample), and a method for calibrating the spectral characteristic measuring apparatus. The present invention also relates to a spectral characteristic measuring system incorporated with a spectrocolorimeter or the like.
2. Description of the Related Art
Generally, a spectral characteristic measuring apparatus such as a spectrocolorimeter performs spectral measurement in a wavelength band from 400 nm to 700 nm or a measurement wavelength region slightly wider than the wavelength band, and at a half bandwidth or a wavelength pitch of 10 nm or 20 nm. Specifically, the spectral characteristic measuring apparatus is provided with a spectral section for separating light into light rays in accordance with wavelengths, and a light receiving section constituted of an array of light receiving elements for outputting electrical signals in accordance with an intensity of received light. The light rays separated by the spectral section in accordance with wavelengths are incident onto the respective light receiving elements, and electrical signals in accordance with the intensities of the light rays received on the light receiving elements are outputted from the light receiving elements.
In the spectral characteristic measuring apparatus, a performance error may occur at a production stage due to a variation in performance of the parts such as the spectral section and the light receiving elements. Further, arrangement positions of the parts or assembly error may be a factor of measurement error. Even if an identical sample is measured, measurement values to be obtained at a final stage may vary. In view of this, normally, spectral characteristic measuring apparatuses are individually calibrated before shipment.
Calibration of a spectral characteristic measuring apparatus is performed by obtaining spectral sensitivities of light receiving elements and storing the spectral sensitivities in the spectral characteristic measuring apparatus. At the time of measurement, accurate values are obtained by interpolating measurement values with use of the stored values. For instance, in calibration at the time of shipment, central wavelengths of the light receiving elements are obtained and stored in the spectral characteristic measuring apparatus. Then, at the time of measurement, performing a computation based on measurement values obtained by the measurement and the stored central wavelengths enables to obtain an accurate spectral reflection characteristic of a sample.
In the following, there is described a method for obtaining central wavelengths of the light receiving elements in calibration. Specifically, monochromatic light rays of different wavelengths are successively incident onto a spectral characteristic measuring apparatus by a spectral illuminator capable of outputting monochromatic light rays each having a sufficiently small half bandwidth at each of wavelengths, and electrical signals to be outputted from the light receiving elements are plotted. Thereby, central wavelengths of the light receiving elements are obtained. Each of the central wavelengths may be defined as a centroid or a peak of a spectral sensitivity characteristic of each of the light receiving elements.
For instance, patent document 1 recites an arrangement, wherein central wavelengths are obtained in calibration, integrated values of spectral sensitivities in a range including each central wavelength as a reference value are stored, and a computation is performed using measurement values obtained by measurement and the stored values to improve precision on measurement values. Patent document 2 recites an arrangement, wherein a central wavelength (a peak wavelength) of each of the light receiving elements is obtained and stored in advance in calibration, and measurement values obtained by measurement are subjected to interpolation based on the central wavelengths to improve precision on measurement values.    Patent document 1: JP Sho 62-289736A    Patent document 2: JP Sho 62-284226A
Conventionally, a xenon lamp, a tungsten lamp, or a like device has been used as an illuminating section in a spectral characteristic measuring apparatus. In recent years, a white LED (Light Emitting Diode) or a like device has been used as an illuminating section, in place of a xenon lamp or a tungsten lamp. Whereas a spectral intensity distribution of a xenon lamp or a tungsten lamp with respect to wavelengths is relatively flat, a spectral intensity distribution of a white LED with respect to wavelengths is sharp. In the specification, a flat spectral intensity distribution means a small change in spectral intensity between adjacent wavelengths of a light receiving element array; and a sharp spectral intensity distribution means a large change in spectral intensity between adjacent wavelengths of a light receiving element array.
The above feature is described in detail referring to FIGS. 9 and 10. FIG. 9 is a graph showing a spectral intensity distribution of a tungsten lamp with respect to wavelengths. FIG. 10 is a graph showing a spectral intensity distribution of a white LED with respect to wavelengths. Referring to FIGS. 9 and 10, the axis of abscissa indicates a wavelength, and the axis of ordinate indicates a relative spectral intensity. As shown in FIG. 9, as the wavelength of light from the tungsten lamp is increased, the relative spectral intensity of light from the tungsten lamp is increased. However, there is no specific portion where the relative spectral intensity of light from the tungsten lamp is sharply increased. The relative spectral intensity of light from the tungsten lamp is monotonously increased. Thus, it is conceived that the spectral intensity distribution of the tungsten lamp is substantially flat.
On the other hand, in the example shown in FIG. 10, the spectral intensity distribution of the white LED has two peaks substantially in the same wavelength band as FIG. 9. In other words, the spectral intensity distribution of the white LED has plural spectral intensity peaks in a predetermined wavelength band, i.e. a portion where the spectral intensity is sharply increased and a portion where the spectral intensity is sharply decreased with respect to wavelengths, as compared with the spectral intensity distribution of the tungsten lamp shown in FIG. 9. Thus, it is conceived that the spectral intensity distribution of the white LED shown in FIG. 10 is sharp.
In a spectral characteristic measuring apparatus, light reflected from a sample irradiated with illumination light is separated in accordance with wavelengths for measurement. Accordingly, measurement values contain a characteristic of the illumination light. In the case where a tungsten lamp is used as an illuminating section as in the conventional art, it is possible to determine spectral sensitivities of the light receiving elements of the light receiving section of the spectral characteristic measuring apparatus in consideration of the performance of the light receiving elements only. However, as described above, in the case where a white LED having a sharp spectral intensity distribution with respect to wavelengths is used as an illuminating section, spectral sensitivities of the light receiving elements are strongly influenced by the spectral intensity distribution of the illumination light from the illuminating section. In the following, an influence of illumination light on spectral sensitivities is described.
Spectral sensitivities in a spectral characteristic measuring apparatus are described referring to FIGS. 11A through 14B. FIGS. 11A and 11B are graphs showing spectral sensitivities of light receiving elements, in the case where a tungsten lamp is used as an illuminating section. FIG. 11A is a graph showing spectral sensitivities of light receiving elements, and a spectral intensity distribution of illumination light. FIG. 11B is a graph showing combined sensitivities of the light receiving elements. FIGS. 12A and 12B are graphs showing spectral sensitivities of light receiving elements, in the case where a white LED is used as an illuminating section. FIG. 12A is a graph showing spectral sensitivities of light receiving elements, and a spectral intensity distribution of illumination light. FIG. 12B is a graph showing combined sensitivities of the light receiving elements. FIGS. 13A and 13B are graphs showing spectral sensitivities of light receiving elements, in the case where a tungsten lamp is used as an illuminating section. FIG. 13A is a graph showing combined spectral sensitivities of light receiving elements. FIG. 13B is a graph showing a weighted combined sensitivity. FIGS. 14A and 14B are graphs showing spectral sensitivities, in the case where a white LED is used as an illuminating section. FIG. 14A is a graph showing combined spectral sensitivities of light receiving elements, and FIG. 14B is a graph showing a weighted combined sensitivity. In FIGS. 11A through 14B, the axis of abscissa indicates a wavelength, and the axis of ordinate indicates a relative spectral sensitivity of each of the light receiving elements, or a relative spectral intensity of illumination light. In FIGS. 11A, 11B, 12A, and 12B, the light receiving elements No. 1 through No. 3 respectively indicate three light receiving elements arranged adjacent to each other, wherein the light receiving elements No. 1 through No. 3 are arranged in this order.
In the case where a tungsten lamp is used as an illuminating section, spectral sensitivities of the light receiving elements, and a spectral intensity distribution of illumination light, as shown in FIG. 11A, are obtained. As shown in FIG. 11A, the spectral intensity distribution of the illumination light from the tungsten lamp is substantially flat. Further, the spectral sensitivities of the light receiving elements shown in FIG. 11A are values free from an influence of illumination light. Since illumination light affects spectral sensitivities in actual measurement, combined sensitivities of the light receiving elements in actual measurement show the values as shown in FIG. 11B. Specifically, each of the combined sensitivities is obtained by multiplying a spectral sensitivity of each of the light receiving elements with a spectral intensity of illumination light. As is obvious from comparison between FIGS. 11A and 11B, central wavelengths of the light receiving elements No. 1 through No. 3 are substantially the same between the graphs of FIGS. 11A and 11B.
In the case where a white LED is used as an illuminating section, spectral sensitivities of light receiving elements and a spectral intensity distribution of illumination light, as shown in FIG. 12A, are obtained. As shown in FIG. 12A, the spectral intensity distribution of the illumination light from the white LED is sharp, and a gradient thereof is large, as compared with the case of the tungsten lamp. Similarly to the graph of FIG. 11A, the spectral sensitivities of light receiving elements shown in FIG. 12A are values free from an influence of illumination light. FIG. 12B shows values of combined sensitivities including an influence of the illumination light. As is obvious from comparison between FIGS. 12A and 12B, central wavelengths of the light receiving elements No. 1 through No. 3 are displaced from each other between the graphs of FIGS. 12A and 12B. Specifically, in the case where the spectral intensity of the illumination light is sharply increased, as the wavelength is increased, the central wavelengths of the light receiving elements No. 1 through No. 3 based on combined sensitivities are increased, as compared with the central wavelengths calculated solely based on the performance of the light receiving elements.
As described above, in the case where a tungsten lamp is used as an illuminating section, there is no or less influence of illumination light. However, in the case where a white LED or a like device having a sharp spectral intensity distribution with respect to wavelengths is used as an illuminating section, it is difficult to obtain accurate measurement values by storing central wavelengths free from an influence of illumination light, and interpolating measurement values with use of the stored central wavelengths.
There is also proposed a method for improving precision on spectral sensitivities of light receiving elements, and improving the S/N ratio by performing a weighting calculation with respect to spectral sensitivities of a certain light receiving element and light receiving elements adjacent thereto. In performing this method, in the case where a tungsten lamp is used as an illuminating section, spectral sensitivities of a middle light receiving element (light receiving element No. 2) and the light receiving elements (light receiving elements No. 1 and No. 3) adjacent to the middle light receiving element, as shown in FIG. 13A, are obtained. The combined sensitivities of the light receiving elements shown in FIG. 13A correspond to the combined sensitivities of the light receiving elements shown in FIG. 11B. As shown in FIG. 13A, since the combined sensitivities of the light receiving elements No. 1 and No. 3 adjacent to the light receiving element No. 2 are substantially identical to each other, weighting coefficients to be applied to the light receiving elements No. 1 and No. 3 for weighting calculation may be identical to each other. Performing a weighting calculation by using the combined sensitivities of the light receiving elements No. 1 through No. 3 shown in FIG. 13A enables to obtain a weighted combined sensitivity as shown in FIG. 13B. As is obvious from comparison between FIGS. 13A and 13B, the combined central wavelength shown in FIG. 13B is substantially the same as the combined central wavelength of the light receiving element No. 2 shown in FIG. 13A, and there is substantially no displacement between the combined central wavelengths in FIGS. 13A and 13B. The combined central wavelength is a central wavelength of a light receiving element calculated based on a combined sensitivity.
Next , described is a case where a white LED is used as an illuminating section. In the case where a white LED is used as an illuminating section, combined sensitivities of the middle light receiving element (light receiving element No. 2), and the light receiving elements (light receiving elements No. 1 and No. 3) adjacent to the middle light receiving element, as shown in FIG. 14A, are obtained. The combined sensitivities of the light receiving elements shown in FIG. 14A correspond to the combined sensitivities of the light receiving elements shown in FIG. 12B. Performing a weighting calculation with respect to the combined sensitivities of the light receiving elements shown in FIG. 14A, using proper weighting coefficients enables to obtain a weighted combined sensitivity shown in FIG. 14B. As is obvious from comparison between FIGS. 14A and 14B, the combined central wavelength shown in FIG. 14B is larger than the central wavelength of the light receiving element No. 2 shown in FIG. 14A, and is displaced from the central wavelength of the light receiving element No. 2 shown in FIG. 14A.
Thus , in the case where a white LED is used as an illuminating section, in place of a tungsten lamp, since an influence of illumination light from the white LED is involved in actual spectral characteristic measurement, as described above, it is impossible to obtain accurate measurement values by: obtaining central wavelengths solely based on spectral sensitivities of the light receiving elements free from an influence of illumination light; and interpolating measurement values using the central wavelengths at the time of measurement.